The ability of cells to respond to each other (cell social behavior) is an important part of normal growth and development. Failures in these responses are believed to be involved in the loss of contact inhibition of growth, a form of cell social behavior displayed by tumor cells in vivo and in vitro. The mechanisms by which cell social behavior is regulated remains unknown. One common model for the regulation of cell social behavior is that they are mediated by cell surface carbohydrate, in the form of glycolipid and/or glycoprotein. I have proposed that heparan sulfate (HS), a member of the third class of carbohydrate found on the cell surface, the glycosaminoglycans, is involved in the expression of cell social behavior. This proposal is based, in part, upon my observation that there is a change in the behavior of cell surface HS on DEAE-cellulose as a result of transformation of mouse cells by either DNA (Papova-) or RNA (Leuko-) tumor viruses. Specifically, I have proposed that the cell surface HS forms a network with itself or other surface molecules and that this network acts as both a cellular girdle and a mechanism for intercellular communication. My current research is directed towards critically testing this proposal as well as delineating the transformation-dependent changes that occur in cell surface HS.